The proposed studies are designed to study the natural history and pathogenesis of bronchial asthma and other forms of reversable reactive airway disease in childhood. It is hoped to examine the immunologic mechanisms in the respiratory tract, and the possible role of respiratory viral infections and virus specific immune responses in the development of reactive airway disease. The proposed studies will be conducted in cohorts of normal infants followed from birth to 5-10 years of age, subjects with bronchiolitis and croup, patients with bronchial asthma, and after respiratory syncytial virus infection in monkeys. A number of virologic, immunologic techniques and pulmonary function tests will be employed to determine the relationship of infections and host immune responses to the development of reactive airway disease.